1. Field of the Invention
The present invention relates to an apparatus and method for the discovery and definition of chemical and geochemical anomalies on the surface of the earth and in the subsurface, and more particularly, to an apparatus and method for detecting and mapping volatile substances associated with geochemical and chemical anomalies from petroleum and natural gas, metallic and non-metallic minerals, chemical wastes, and geothermal resources, wherein volatile substances from these anomalies migrate to the earth's surface.
2. The Prior Art
Historically, geochemical prospecting for petroleum and natural gas using hydrocarbons detected at the earth's surface has been based primarily on C.sub.1 -C.sub.5 hydrocarbons with special emphasis on methane (C.sub.1). In most cases, a sample of gaseous hydrocarbons from soil gas is collected via a probe driven 3 to 5 feet into the ground or, alternatively the soil gas sample is collected from a 8- to 10-foot hole in the ground and subsequently analyzed by gas chromatography. Alternatively, a hole is prepared and sealed by a packer after the sample collector has been inserted in the hole. The gas sample is collected after a suitable equilibration period. Since these samples are collected over a very short period of time they are commonly called instantaneous or "grab" samples. The high vapor pressure of volatile substances (C.sub.1 -C.sub.5) results in their being present in a high enough concentration in the soil gas to be detected by instantaneous sampling. Volatile substances of low vapor pressure, which include most substances of high molecular weight (above C.sub.5), cannot be detected by the instantaneous methods.
The individual C.sub.1 -C.sub.5 hydrocarbons are subjectively used to define anomalies. The adsorbed compounds are chemically desorbed and analyzed for the concentration of these light hydrocarbons. The relative concentrations, and to a lesser extent, ratios of individual concentrations are used in a non-computer analysis to define anomalies.
The use of an instantaneous sample is strongly influenced by common interferences such as meteorology, soil temperature, soil type, moisture content, biological activity and other phenomena. Numerous publications cite the importance of these interferences on the measured concentrations of hydrocarbons and other gases: Representative literature references include the following:
Ryall, W. R. Guidelines for collection, preparation and storage of geochemical samples for mercury analysis. Tech. Comm.-CSIRO Inst. Earth Resources, No. 67, 17 pp., 1979.
Kraner, H. W., Measurements of the effects of atmospheric variables on the Rn-222 flux and soil gas concentration. In The Natural Radiation Environment. J. A. S. Adams, ed., U. Chicago Press, pp. 191-215, 1964.
Tanner, A. B., Radon migration in the ground. In The Natural Radiation Environment, J. A. S. Adams, ed. Univ. Chicago Press, pp. 161-190, 1964.
Tanner, A. B. Radon migration in the ground: A supplementary review. U.S. Geol. Survey, Open-file Rept. 78-1050, 62 p, 1978.
Reimer, G. M., Helium detection as a guide for uranium exploration. U.S. Geol. Survey, Open-file Rept., 76-240, 14 p., 1976.
Klusman, R. W. and Webster, J. D., Meteorological noise in crustal gas emission relevant to geochemical exploration. J. Geochem. Explor. v. 15, pp. 63-76, 1981.
Klusman, R. W., Variations in mercury and radon emission at an aseismic site. Geophys. Res. Lett., v. 8, pp. 461-464, 1981.
Ball, T. K., Nicholson, R. A., Peachey, D., Effects of meteorological variables on certain soil gases used to detect buried ore deposits. Trans., Inst. Mining, Metallurgy, London, v. 92, pp. B183-B190, 1983.
Adams, D. F., Farwell, S. O., Pack, M. R. and Robinson, E., Biogenic sulfur gas emissions from soils in eastern and southeastern United States. J. Air Pollut. Control Assoc., v. 31, pp. 1083-1089, 1981.
Blackmer, A. M., Diurnal variability in rate of emission of nitrous oxide from soils. Soil Sci. Soc. Amer. J., v. 46, pp. 937-943, 1982.
Rightmire, C. T., Seasonal variation in P.sub.CO2 and 13.sub.C content of soil atmosphere. Water Resource. Res., v. 14, pp. 691-692, 1978.
Whitehead, D. C., The volatilization, from soils and mixtures of soil components of iodine added as potassium iodide. J. Soil Sci., v. 32, pp. 97-102, 1981.
Klusman, R. W. and Landress, R. A., Secondary controls on mercury in soils of geothermal areas. J. Geochem. Explor., v. 9, pp. 75-91, 1978.
Klusman, R. W. and Landress, R. A., Mercury in soils of the Long Valley, Calif. geothermal system. J. Volcan. Geothermal Res., v. 5, pp. 49-65, 1979.
Klusman, R. W., Cowling, S., Culvey, B., Roberts, C., Schwab, A. P., Preliminary evaluation of mercury and arsenic in soils of selected Colorado geothermal districts. Geothermics, v. 6, pp. 1-8, 1977.
Trost, P. B. and Bisque, R. E., Differentiation of vaporous and ionic mercury in soils. Int. Geochem. Explor. Symp., Toronto, pp. 276-278, 1971.
Swaby, R. J., Emission of sulfur to the atmosphere from soil. Sulfur Aust. Pop., Workshop, pp. 14-19, 1978, (pub. 1980).
Ryall, W. R., Guidelines for collection, preparation and storage of geochemical samples for mercury analysis. Tech. Comm. -CSIRO Inst. Earth Resources, No. 67, 17 pp., 1979.
Klusman, R. W. and Matoske, C. P., Adsorption of mercury by soils from oil shale development areas in the Piceance Creek basin of northwestern Colorado. Environ. Sci. Technol., v. 17, pp. 251-256, 1983.
Hunt, J. M., "Petroleum Geochemistry and Geology" San Francisco, W. H. Freeman, 617 p. 1979.
To overcome the meteorologic and biologic influences on instantaneous sampling, an integrative or continuously collecting medium is sought. Soil itself has been used to integrate volatile substances in the application to petroleum exploration. Soils are complex and heterogeneous materials whose ability to retain and integrate volatile substances is highly variable. Factors such as clay mineralogy, organic matter content, pH, climate, plant cover, and other factors influence the retention capability for volatile substances. Some of these problems are addressed for a wide variety of volatile substances by various authors:
Adams, D. F., Farwell, S. O., Pack, M. R. and Robinson, E., Biogenic sulfur gas emissions from soils in eastern and southeastern United States. J. Air Pollut. Control Assoc., v. 31, pp. 1083-1089, 1981.
Blackmer, A. M., Diurnal variability in rate of emission of nitrous oxide from soils. Soil Sci. Soc. Amer. J., v. 46, pp. 937-943, 1982.
Rightmire, C. T., Season variation in P.sub.CO2 and 13.sub.C content of soil atmosphere. Water Resource. Res., v. 14, pp. 691-692, 1978.
Whitehead, D. C., The volatilization, from soils and mixtures of soil components of iodine added as potassium iodide. J. Soil Sci., v. 32, pp. 97-102, 1981.
Klusman, R. W. and Landress, R. A., Secondary controls on mercury in soils of geothermal areas. J. Geochem. Explor., v. 9, pp. 75-91, 1978.
Klusman, R. W. and Landress, R. A., Mercury in soils of the Long Valley, Calif. geothermal system. J. Volcan. Geothermal Res., v. 5, pp. 49-65, 1979.
Klusman, R. W., Cowling, S., Culvey, B.., Roberts, C., Schwab, A. P., Preliminary evaluation of mercury and arsenic in soils of selected Colorado geothermal districts. Geothermics, v. 6, pp. 1-8, 1977.
Trost, P. B. and Bisque, R. E., Differentiation of vaporous and ionic mercury in soils. Int. Geochem. Explor. Symp., Toronto, pp. 276-278, 1971.
Swaby, R. J., Emission of sulfur to the atmosphere from soil. Sulfur Aust. Pop., Workshop, pp. 14-19, 1978, (pub. 1980).
Ryall, W. R., Guidelines for collection, preparation and storage of geochemical samples for mercury analysis. Tech. Comm. -CSIRO Inst. Earth Resources, No. 67, 17 pp., 1979.
The last author indicated stringent storage requirements may have to be met for soils to retain volatile substances. Another publication demonstrated the ability of a group of soils to retain volatile substances increases and decreases with season.
Klusman R. W. and Matoske, C. P., Adsorption of mercury by soils from oil shale development areas in the Piceance Creek basin of northwestern Colorado. Environ. Sci. Technol., v. 17, pp. 251-256.
To overcome the problem in using soils as an integrating medium for volatile substances, a synthetic material such as charcoal, resins, molecular sieve, or other material has been used as a collector. Charcoal has been described as a method for trapping of gases from underground petroleum accumulations (U.S. Pat. Nos. 2,266,556; 2,284,147). Appropriate containers for underground collection were described in these cases. However, no description of analysis of the trapped hydrocarbons and no description of data interpretation were presented.
Other volatile organic and inorganic substances are potentially useful in the exploration for mineral deposits and geothermal resources. The volatile substances which have been described in the literature as being useful in these applications include: sulfur gases, mercury, radon, helium, arsenic-containing gases, carbon dioxide, and halogen-containing gases, while hydrocarbons have been used to a lesser extent. The same problems associated with instantaneous methods and using soil as an integrative medium apply as well to these applications.
A wide variety of toxic and hazardous substances have been dispersed in soils and groundwater by human activities. Many of these are hydrocarbon-based or halogenated hydrocarbon-based. Most of these compounds are adsorbed by soils and have sufficient volatility to be detected by integrative collection methods. In this geologic environment, the complex interactions previously described exert their influence and make data interpretation difficult.
The detection of minute volatile hydrocarbon samples from non-geologic sources use an analytical technique based on Curie-point pyrolysis. This technique is disclosed in a publication entitled "Novel Method for the Direct Analysis of Hydrocarbons in Crime Investigations and Air Pollution Studies," J. D. Twibell, Janet M. Home, Nature, Vol. 268, 25 August 1977. In this publication, and in at least two other articles, a procedure using charcoal glued to a ferromagnetic wire is disclosed for use in conjunction with gas chromatography as the analytical technique. The procedure overcomes many of the inherent problems of desorbing the volatile substances of interest from the charcoal that all other charcoal adsorption techniques have encountered. By using a ferromagnetic wire as a support, the wire can also serve as a heater to desorb the adsorbed chemicals when placed in a high frequency electromagnetic field.
Prior to the present invention, whether the method used an instantaneous or integrative sample approach, analysis was primarily by gas chromatography and resulted in definition of anomalies where high concentrations of singular volatiles were observed. Methane has been most extensively used of all the hydrocarbons. In many cases, biogenic activity has caused increased methane concentrations which were not associated with a petroleum or thermogenic gas accumulation. Another difficulty is that areas of high volatile flux may represent zones of high permeability (i.e., faults) not necessarily related to deposits of economic interest. Volatile flux data can result in many false anomalies.
Strong warnings exist concerning the usage of the C.sub.1 -C.sub.5 hydrocarbons in defining the boundaries of an accumulation. The primary accepted use of the described methodology is to select general areas for drilling but not to give an outline of the area encompassed by the hydrocarbon accumulation. The problems of using volatile materials and volatile materials adsorbed by soils are not trivial. The widely accepted primary textbook, in a summary on seeps and surface prospecting, states,
"Vertical diffusion of hydrocarbons from subsurface petroleum accumulations is not the mechanism that causes surface hydrocarbon anomalies. These anomalies are probably caused by the diffusion of gases from decaying organic matter in the first few hundred feet of burial and by hydrocarbons migrating to the surface by mechanisms other than diffusion. Buoyancy is the most likely driving mechanism. PA0 Surface geochemical prospecting cannot outline oil or gas accumulations at depth except in rare cases. It can be useful as an auxiliary prospecting tool providing that data on subsurface geology, near surface hydrocarbons, and fluid flow systems in the sediments is available. It is most useful where intrusions, fault or fracture systems, or permeable beds are providing vertical pathways of migration by processes other than diffusion. In such regions, it can assist in differentiating structures and areas that contain hydrocarbons from those that are barren." (Page 433)
Hunt, J. M., "Petroleum Geochemistry and Geology" San Francisco, W. H. Freeman, 617 p. 1979.
The present invention eliminates the previously described problems and presents new methodology for the collection and analysis of hydrocarbons, organics, and certain inorganic volatile species. It also provides an objective methodology for the computer interpretation of the data and the production of a unique map which outlines the boundary of the accumulation. The present invention uses an integrative sampling approach with various adsorbents which eliminates the need for correction for such parameters as soil temperature, meteorology, biologic effects, etc. This present invention also eliminates the problems of gas diffusion into the container and expands the number of hydrocarbon compounds used in the analysis from C.sub.1 -C.sub.5 to C.sub.2 -C.sub.18+. Methane is not used with the present technology. Other organic and inorganic compounds are also collected and analyzed by this invention.
Definition of the multiplicity of compounds adsorbed on the adsorbent is done by any suitable device which generates a characteristic, diagnostic, and discriminatory fingerprint. Multivariate statistics are used to objectively classify the various fingerprints. A geochemical model used as a "training set" is an important aspect of the data analysis and allows for clear definition and characterization of underground accumulations. Finally, unique maps are produced which show a quantitative similarity of a sample set to the characteristic geochemical model.